This error will appear if the SWATH configuration file is not edited to allow for window widths below 4 Da.
The file location is: C:\Users\<User Name>\AppData\Roaming\AB SCIEX\PeakView 2\SwathUserSettings.xml
To modify this file, right clicki on the .xml file and open it in Notepad. Find the following entry in the file: <SwathWindowBelow4Da value="True" /> (Originally, it will say "False" by default, but change it to "True".)
Save the Notepad file as .xml and close it.